It has been shown that a number of human and rodent leukemic cell lines are dependent on preformed cysteine in tissue culture media and that they remain growth arrested in the presence of cystathionine, the precusor of cysteine. Substantial evidence exists to suggest that the cysteine auxotrophy of such cells (cys-) results in part from a depletion of cystathionase, the enzyme responsible for the conversion of cystathionine to cysteine. Human lymphoblastoid cell lines derived from nonleukemic individuals maintain normal growth in the absence of cystein, provided cystathionine is present and they have easily detectable cystathionase (cys+). I have recently shown that cysteine auxotrophy may be expressed in vivo by leukemic cells. I have developed a murine hybridoma cell line which secretes a monospecific antibody to human cystathionase. A major objective of the proposed research will be to use my reagent to quantify the content of cystathionase in fresh human leukemic, lymphoma, and solid tumor cells and compare the levels to those of the relevant normal tissues. The techniques used will include immunofluorescence and immunoperoxidase staining of relevant cells as well as rocket immunoelectrophoresis and radioimmunoassay. A second objective will be to detemine the sensitivity of the earliest committed stem cells from human bone marrow (CFU-C) to cysteine depletion in the presence and absence of cystathionine. The sensitivity of such cells will be compared to their cystathionase content, and the expression of cystathionase in the various stages of the maturing normal granulocyte and gut epithelial cell will be delineated. Finally, we propose to explore the relative toxicity of various cysteine analogues to both cys+ and cys- cells. The potential for selective cystathionine "rescue" of cys+ cells in the presence of toxic cystein analogues will be examined by in vitro tissue culture techniques.